The contact between the tubular invagination of the surface membrane and the sarcoplasmic reticulum is believed to have unused electrophysiological properties. Its ultrastructure will be determined with the assistance of a computer coupled microdensitometer and correlated with its electrophysiological properties. The mechanism of substrate inhibition in muscle will be explored in skinned crayfish and human muscle and several models for this tested. We plan to compare the predictions of the models by a morphological method and with tension data collected in high concentrations of substrate and Ca, as well as in the presence of added myosin fragments. The in vivo structural lability of the Z band will be investigated.